Rigging a pencil case
Maybe you should try to rig with the Stretch To constraint, I give here an explanation for an arm (method #2), it's exactly the same for your pen case, except you'll need several bones all around the object. Also you may want more than 2 segments if the deformation is more complicated:

You may also want to try Spline IK but it's a bit tedious, so if Stretch To works fine, then use it.